For the measurement of the oil temperature of a transformer and a tap changer, for example, the use of a resistance thermometer is common. Such resistance thermometers are available from a number of manufacturers and have generally standard dimensions and construction. In Germany, for example, the resistance thermometer must conform to the German Industrial Standard DIN 42554 and in other countries similar standards are applicable. In the United States and Canada, for instance, standard screw threads such as 3/8 inch NPT or 7/8-14 UNF are customary. The resistance thermometers which can be used are those made by Messko Hauser, Thermometerwerk Geraberg and Sika GmbH, all of Germany, and a Qualitrol Corp., of New York in the United States. Qualitrol supplies, for oil-filled power transformers and tap changers, thermometers of the Series 165/167, 104, 104-400. These units are described in the Qualitrol Corp. brochures QT-150, QT-165, QT-104-2, QT 104-321 and QT-104-400. Such units can have a housing or body provided with the electronic circuitry and a stem in which the resistor is housed, a screw thread being provided adjacent a prismatic formation which can be engaged by a wrench. The housing may be provided with connectors allowing cabling to electronic circuitry.
The thermometer is normally threaded into a thermometer pocket of the apparatus and can have a lower portion as a perforated protected tube in which a replaceable sensor insert is received. A vent can be provided on the housing as well.
Resistance thermometers of the aforementioned type have a number of drawbacks in the form known from the aforementioned brochures and the housing can have three or four conductor connectors as may be required or desirable.
Because of the contact resistances, the use of electromagnetic interference circuitry and the like, the measurement results of such systems can be falsified, especially as a result of time constants of various components of the electromagnetic interference circuitry. Such circuitry is frequently required to prevent interference from electromagnetic environments.
The output of the resistance thermometer frequently requires additional electronic processing to produce an output which can be provided to a display device or which is suitable as an input to a computer. This is a consequence of the fact that the output signals of some of the known resistance thermometers mentioned above are not compatible for computers or such display devices. Conventional resistance thermometers have a resistance variation .DELTA.R which is dependent upon the temperature e so that .DELTA.R=f (.theta.). Electronic devices generally require standard well-defined signal levels, for example in the range of 0 to 10V or 0 to 20 mA or 0 . . . 1 mA. As a result, signal processing is practically always required to provide signals in this range for the downstream electronic units.
Finally, monitoring of the functioning of a resistance thermometer and thus the reliability and precision of the temperature measurement in use is not possible. In the past resistance thermometers have only been capable of being monitored by a separate reference measurement off line.